Thermal transfer, ribbons, also variously called film ribbons, thermocolor ribbons, thermal color ribbons, thermocarbon ribbons, thermal carbon ribbons, or thermally activated image transfer ribbons, have been known for a long time.
They typically have a film-like substrate, also known as a base or carrier, which may consist of paper, plastic foil (film), or some other suitable flat thin substrate, bearing a fusible pigmented layer, this layer typically consisting of colorant substance such as a dyestuff or carbon black bonded by a fusible plastic or a wax. By virtue of this fusible material, the colorant substance may be transferred by way of the melt onto the item to be printed, such as paper, through the action of a printhead which applies heat and pressure to the ribbon.
Thermal printers and thermal/pressure printheads suitable for this process are disclosed for example in German patent documents DE-AS 2 062 494 and DE-AS 2 406 613 as well as in German Application DE-OS 3 224 445.
In greater detail, the process can be described as follows: on a thermal/pressure printhead of the printer, the letter to be printed is formed by heated points. The printhead presses the thermal transfer ribbon against the paper to be printed. The heated letter, having a temperature of about 400.degree. C., causes the fusible colorant layer to melt off and transfer to the paper to be printed. The used part of the thermal ribbon is then carried to a spool.
The thermal transfer ribbon can have various fusible colorants in combination. With the combination of three basic colors, blue, yellow, and red, multicolored printing can be accomplished. In contrast to ordinary color photography, the troublesome steps of developing and fixing can be dispensed with. Thermal printers can be operated at high speed--a DIN (German Industrial Standard)--A4 page can be printed in about 10 seconds--and without producing a troublesome level of noise.
Besides the above described thermal transfer ribbons, there is also a type whereby the thermally printed character is not produced by a thermal/pressure printhead but by resistance heating of an especially-constituted filmlike substrate. The fusible colorant, which is the functional layer here as in the pressure process, also contains the constituents described above. In commerce these devices are called "electrothermal ribbons". This type of thermal printing system is described, for example, in U.S. Pat. No. 4,309,117.
Thermal transfer ribbons are already known which can produce multiple impressions, often called multi-use ribbons. Such ribbons are described for example in European Patent No. A-0 063 000. The fusible colorant of these thermal transfer ribbons is a particulate material, insoluble in the fluid medium used to coat the ribbons, and which does not melt below 100.degree. C., interspersed with an additional particulate material with a melting point between 40.degree. and 100.degree. C. The particulate material which does not melt below 100.degree. C. advantageously consists of a metal oxide, a metal, an organic resin, or carbon black. By means of this special particulate material, the layer of fusible colorant, which itself is a solid mixture, is given a heterogeneous structure, from which only a portion of the transferable molten colorant is expended on each application of pressure.
However, with thermal transfer ribbons of this sort, improvements are needed in print quality, especially with regard to the sharpness of definition of the printing. In this regard, the greatest improvement results from having a sponge-like structure of the fusible colorant as in German Application 35 20 308. This is produced by a special process wherein a coating fluid is applied to the substrate film of the ribbon in the usual way; this coating fluid has dissolved in it a thermoplastic binder and also has dispersed in it a finely divided solid wax or waxlike substance. This coating fluid employs as its solvent medium a mixture of a room-temperature solvent and a non-solvent for the thermoplastic binder. The non-solvent/solvent mixture evaporates off with simultaneous loss of its solvency for the thermoplastic binder. This product, like those described earlier, still does not perform in a totally satisfactory manner for making multiple impressions.